Embodiments of the present invention relates to the correction of the optical response of display systems, and in particular, but non-exclusively, to devices enabling correction of the optical response of flat screen display systems.
The optical response of display systems is not linear. Any image signal intended to be displayed on a screen must be corrected so that the reproduced image exhibits a contrast corresponding to that of the original image.
In FIG. 1, curve 1 illustrates the optical response of a cathode-ray display system, comprising a cathode-ray screen, also called CRT screen. Curve 1 is shown in a coordinate system Ox, Oy and extends from origin O to an end point E. The coordinates of point E are xmax, ymax and ymax=xmax, corresponding to the maximum value that can be taken by a pixel. On abscissa axis Ox are plotted input pixels and on ordinate axis Oy are plotted output pixels.
If a pixel P of value xp is displayed with no correction on the cathode-ray screen, the optical response of the screen is such that the displayed pixel exhibits a value ya smaller than xp. For pixel P once displayed to have a value yp equal to xp, a corrector circuit which modifies the value of the input pixel must be interposed between the input pixel and the display system so that, after display, the value of the displayed pixel corresponds to the value of the input pixel.
Curve 2 of FIG. 1 shows the response curve that the corrector device must exhibit to provide a correct display of the input pixels. Curve 2, which extends from origin O to end point E, is a conventional gamma correction curve very currently used to correct the response of cathode-ray screens.
Liquid-crystal display screens, or LCD screens, exhibit an optical response curve formed of two rounded lobes, called an “S” curve.
FIG. 2 shows, in a referential Ox, Oy corresponding to that of FIG. 1, the ideal response curve that a corrector device should exhibit for an LCD-type screen. This “S” curve is formed of two rounded lobes A and B, joining at an inflexion point I of coordinates xI, yI. It extends from origin O to an end point E of coordinates xmax and ymax=xmax. The position of point I, as well as the curvature of lobes A and B, depend on the considered LCD screen.
Generally, corrector devices of flat screen display systems must exhibit rather complex response curves. “Flat screen display system” means all display systems other than those of CRT type, for example, plasma screen display systems, electroluminescent screens, liquid crystal displays, etc.
Further, if both a CRT-type display system and a flat screen display system are desired to be corrected, different corrector devices must be provided.